The objective of this research project is the investigation of the relationship of amino acid transport to the regulation of growth. Specifically, we are analyzing the development of amino acid transport systems in early mouse embryos. Cleavage stage development of mouse embryos can occur in vitro in the absence of exogenous amino acids. Nevertheless, amino acids are actively transported into mouse embryos as early as the four-cell stage. By the blastocyst stage, the rate of amino acid transport has increased sharply for a number of amino acids studied, and this increase is due largely to the activation of Na ion-dependent amino acid transport systems for methionine and leucine. We view this increased rate of amino acid transport as part of the overall "preparation for growth," which is characteristic of late cleavage staged embryos. The "preparation for growth" period culminates in blastocyst formation which also can occur in the absence of amino acids in vitro. Shortly after blastocyst formation, mouse embryos acquire the need for amino acids in order to undergo successful hatching, attachment, and outgrowth of embryos in vitro. It is during this phase of development that initiation of growth (increase in mass) begins. The attachment and outgrowth of these embryos in vitro is considered to be analogous to the implantation process in vivo. Thus, the acquisition of amino acid requirements coincides closely with the initiation of the growth phase of development, which involves active, Na ion-dependent amino acid transport systems. BIBLIOGRAPHIC REFERENCES: Borland, R. M. and R. J. Tasca (1975). Inhibition of L-methionine uptake and incorporation by chlorpromazine in preimplantation mouse embryos. J. Reprod. Fert. 42, 473-481. Borland, R. M. and R. J. Tasca (1975). Na ion-dependent amino acid transport in preimplantation mouse embryos. II. Metabolic inhibitors and nature of the cation requirement. Develop. Biol. 46, 192-201.